Conventionally, there is known a wiring substrate including plural wiring layers and plural insulating layers alternately layered on a substrate body formed of an inorganic material such as silicon, glass, or ceramic. The wiring substrate also includes adjacent wiring layers that have the insulating layers interposed therebetween and are connected by a via hole penetrating through the insulating layers.
FIG. 1 is a cross-sectional view of a wiring substrate 100 of a related art example. With reference to FIG. 1, the wiring substrate 100 includes a substrate body 110 having a first surface on which a GND (ground) plane layer 130 is formed. The wiring substrate 100 also includes a first insulating layer 160 covering the GND plane layer 130. The substrate body 110 has a second surface on which a power plane layer 140 is formed. The wiring substrate 100 also includes a second insulating layer 170 covering the power plane layer 140. Further, a penetration wiring 150, which penetrates through the substrate body 110, electrically connects between a wiring layer (not illustrated) formed on the first surface of the substrate body 110 and a wiring layer (not illustrated) formed on the second surface of the substrate body 110. It is to be noted that the substrate body 110 may be covered with an insulating film.
The GND plane layer 130 and the power plane layer 140 are formed relatively thick because the GND plane layer 130 and the power plane layer 140 are formed for stabilizing the ground or the electric potential of the power supply. A layer including the term “plane layer” is a layer that is flatly formed almost entirely on a predetermined surface. The ground plane layer 130 is formed almost entirely on the first surface of the substrate body 110 except at the vicinity of an end surface of the penetration wiring 150. The power plane layer 140 is formed almost entirely on the second surface of the substrate body 110 except at the vicinity of an end surface of the penetration wiring 150.
For example, a photosensitive polyimide type resin may be used to form the first and the second insulating layers 160, 170. Wiring layers (not illustrated) are formed on the first and the second insulating layers 160, 170, respectively. By forming a via hole penetrating through the first and the second insulating layers 160, 170, the wiring layers (not illustrated) formed on the first and the second insulating layers 160, 170 can be electrically connected to the penetration wiring 150.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-294383    Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-171048    Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-158537
For example, in order to form the first and the second insulating layers 160, 170, a photosensitive polyimide type resin may be applied to the GND plane layer 130 and the power plane layer 140, and then cured. In many cases, the GND plane layer 130 and the power plane layer 140 are formed thicker than the wiring layers (not illustrated) formed on the first and the second insulating layers 160 and 170. Accordingly, the first and the second insulating layers 160, 170 (which are formed on the GND plane layer 130 and the power plane layer 140) tend to have a concavo-convex surface rather than a flat surface. In a case where concavities and convexities are formed on the surfaces of the first and the second insulating layers 160, 170, it is difficult to form wiring layers on the first and the second insulating layers 160, 170.